Semiconductor devices and surface acoustic wave devices, which are used in various electronic devices such as mobile communication terminal devices in recent years, have been required to be smaller and lower height along with the increasing miniaturization of such electronic devices.
In order to meet this requirement by reducing mounting area and height of the semiconductor devices and the surface acoustic wave devices, flip-chip mounting is often used to connect these elements face-down to a mounting substrate or to a package having bumps such as solder bumps formed thereon.
The semiconductor devices and the surface acoustic wave devices often include an electrode wiring made of aluminum or aluminum alloy. Such an aluminum-based electrode wiring, however, is so poorly wettable to solder that they cannot be well bonded to each other. For this reason, it is necessary to provide a metal film between the solder bumps and the electrode wiring so as to improve the bonding.
Recent environmental issues are accelerating the use of lead-free solder bumps. However, SnAgCu lead-free solder has a higher reflow temperature and a higher Sn content than the conventional PbSn eutectic solder. Since Sn diffuses easily, it is necessary to solve the problem of the diffusion of the metal film and Sn.
The metal film is generally formed by plating, evaporation-liftoff, or the like. Plating is often used in cases where a photo-resist used as a mask can be left on the substrate without being removed after the plating, because the photo-resist needs to have high chemical resistance. Plating is, however, not suitable for the surface acoustic wave devices because the photo-resist damps the vibration of comb-shaped electrodes and cannot be left on the substrate. Plating also involves problems such as difficulties in refining patterns and a treatment of waste fluid.
In view of this, the metal film of the surface acoustic wave devices is suitably formed by evaporation-liftoff. As described above, the use of SnAgCu solder requires a metal film having a high diffusion resistance. Therefore, the metal film needs to contain a thick barrier layer to prevent the diffusion of Sn. A well-known metallic material having an effect of preventing the diffusion of Sn is nickel.
However, if the thick barrier layer is formed of nickel having a high tensile stress, the photo-resist is likely to be cracked or exfoliated during the deposition of the metal film. Furthermore, the stress is concentrated on the interface between the photo-resist and the metal film, so that if a small load is applied from outside, the metal film is easily exfoliated.
On the other hand, it has been proposed to form a diffusion-resistant barrier layer of nickel having a thickness of 500 to 5000 nm by using a photo-resist pattern that is resistant to nickel stress. The photo-resist pattern is made from a special photo-resist material having a high stress resistance. It is likely that damage of the photo-resist can be reduced by making the photo-resist more resistant to the stress of the thick nickel film. However, the metal film is easily exfoliated by an external load due to the residual stress of the metal film formed on the substrate.
One of the prior arts related to the present application is Japanese Patent Unexamined. Publication No. 2003-318212.